New electronic components are being powered with lower and lower voltages (currently 2.5 V and 1.8 V, and soon probably 1.2 V and 0.8 V) and the power requirements, at very low voltages, are increasing and becoming more important with respect to the conventional voltages +/−15 V and +5 V.
The currents drawn are becoming increasingly large since the power consumed by users is still the same or is increasing (capability for larger number of functions).
Voltages below 3.3 V are not distributed and are installed directly on the user boards. The power supply is being displaced as close as possible to the users.
This tendency obliges power supply manufacturers to produce converters generating ever greater ratios between input voltage and output voltage.
The structures used are generally pulse-mode converters that are not isolated in order to maintain high efficiencies and converters with low dimensions. It is difficult for these converters, with a structure of the step-down type, to achieve a transformation ratio greater than 10 with efficiencies greater than 90%.
In order to satisfy the demands of the market for higher levels of integration, these new converters must be deliverable within smaller and smaller surface areas and hence with increasingly higher efficiencies so as not to increase the size of the power dissipators.
The buck converter is one of the various converter structures.
FIG. 1a shows a functional circuit diagram of a buck converter.
The circuit in FIG. 1a is supplied by an input DC voltage Vin and delivers an output voltage Vout onto a load Rout in parallel with a capacitor Cout. A switch 10 allows either the positive potential of the input voltage Vin or the negative potential to be applied, for respective times Ton and Toff, to a terminal of an output inductor Lout that is connected by its other terminal to one of the load resistance terminals Rout. FIG. 1b shows the closed time Ton and the open time Toff of the switch 10. The other terminal of the load resistance Rout is connected to the negative potential of the input voltage Vin. It will be assumed in the following that the negative potential of Vin is 0 volts.
The diagrams in FIGS. 1c, 1d and 1e show the operational principle of the buck converter.
It is assumed that the switch 10 is switched with a frequency of period T, with T=Ton+Toff (see FIG. 1C). The period T can be a constant or variable value.
The voltage VI across the terminals of the inductor Lout is:
VI=Vin−Vout, during the time Ton and
VI=−Vout, during the time Toff.
The mean voltage Vm of the output voltage Vout across the terminals of the resistance Rout will therefore be in the range between Vin and 0 volts depending on the duty cycle Toff/T and will be given by Vm=(Ton/T).Vin.
The mean value Vm of the voltage Vout is constant. The current Ilout in the inductor Lout takes the form of ramps during the times Ton and Toff. A diode D ensures the continuity of the current in the inductor during the switching operations.
In the diagram in FIG. 1c the case of Ton=T/2 and hence Vout=Vin/2 is shown.
The diagrams in FIGS. 1d and 1e respectively show two values of mean voltage Vm1 and Vm2 across the terminals of the load resistance Rout for two values of the time Ton:                in the diagram in FIG. 1d: Ton/T=0.9 and,        in the diagram in FIG. 1e: Ton/T=0.1.        
In other words, when Ton/T is small, the energy supplied by the power source, during the short time Ton, is small, producing a low mean voltage across the terminals of the load. On the other hand, when Ton is close to the period T, the load is virtually continuously connected to the power source, the mean output voltage is close to the DC input voltage.
In another type of operation of the buck converter, the time Ton is kept constant and the switching frequency, in other words the switching period T, is changed so that the ratio Ton/T is made to vary.
In practice, the switches are formed by two semiconductors in series, for example two MOS switches controlled by complementary signals at the frequency 1/T.
The buck converters of the prior art nevertheless have limitations. Indeed, a duty cycle Ton/T of 0.1 is the minimum that can currently be obtained with an acceptable performance in terms of efficiency and reliability. However, when it is desired to obtain an output voltage lower than one tenth of the input voltage, the conduction time Ton of the semiconductor supplying the energy to the load becomes very short and the switches become very difficult to control. In addition, if the output voltage decreases, for a given power delivered to the load, the currents in the semiconductors become large, at the limits of their capabilities, with a loss of efficiency of the converter.
Another means for obtaining a ratio between the input voltage and the output voltage that is much higher than 10 consists in forming a voltage step-down device comprising two cascaded converters. In this device, the output voltage of a first converter is applied to the input of a second converter. Thus, much higher ratios between the input voltage and the output voltage of the device can be obtained than those obtained by a single converter. Nevertheless, such a step-down device comprising two cascaded converters exhibits a globally lower efficiency than that of a single converter and a higher cost of production.